Soot production in a tubular gas turbine combustor

Abstract

Soot production in gas turbine combustors is not desirable since it is the major source
of exhaust smoke emission and its thermal radiation to the combustor liner deteriorates the liner
durability. Soot formation involves comparatively slow chemistry and equilibrium can not be
applied to soot modelling in the combustor flow field.
. The exact sooting process in the
combustor is poorly understood given both the complexity and the limited experimental data
available. The work reported in this thesis seeks to first develop in-situ techniques for
retrieving spatially-resolved soot properties, mainly soot particle volume fraction, from within
the combustor and also to apply the measured results to comparisons with predicted soot
concentrations.
Two probing methods have been demonstrated which also incorporate a laser absorption
technique. The sight probe proves to be more reliable in the present measurements. The
evaluation of the physical probing techniques in sooty laboratory flames reveals that the flame
structure will not be substantially distorted by the probe. The disturbance caused by the probe
is localised, a feature which is evident in the reported water flow visualization test. The
necessary inert gas purge can be minimised to reduce the local aerodynamic perturbation. The
measured soot volume fraction distributions are comparable with sooting levels reported in
flame studies in the literature. The peak soot volume fractions are located off-axis,
characteristic of the fuel atornization. The measurementsin the primary zone are restricted by
the multi-phase character of the flow, where soot absorption can not be readily discriminated
from fuel droplet scattering. Measurements are reported over a range of air-fuel ratios, inlet
pressures and temperatures.
Time-averageds calard istributionsa t the nominald ilution sectionh ave beeno btained
in addition to the soot measuremenut sing probe sampling and standard gas analysis.
Correlationso f carbond ioxide with mixture fraction reveala clear relationshipa t overall lean
conditionsc onsistenwt ith widely usedm odelleda ssumptions.T here are less well-correlated
relationshipsb etweent emperaturea ndm ixture fraction, possiblyd ue to the influenceo f scalar
fluctuationsa nda lsoo f the scalard issipationr ate. Sootl oadingi n the presentf low conditions
is characteristicallylo w, basedo n the mixture fraction ands ootv olumef raction data. Thermal
radiation in the visible spectrum shows a distinct narrow band spectra in addition to the soot
continuum, which is believed to arise fromC2radical emission. The mean radiation intensities,
predictedb y usingt he measuredte mperaturea nds ootc oncentrationre sults,a rei n generallo wer
than the measured mean intensities. Temperature fluctuation levels may be particularly
influential in some of these calculations.
Sootm odellingi n the combustohr asb eenu ndertakenb y applyinga n extendedla minar
flamelet concept. The two-equations oot formation model has beenp rimarily developedo n
laminar flames. The comparisono f the computationa nd measuremenstu ggeststh at this soot
model holds promise in the context of prediction in the combustor. In the absenceo f a
satisfactoryt heoreticald escriptiono f the fuel-air burning in the combustor,w heret he liquid
kerosinee mployedis replacedb y gaseoups ropane,t he computeds calarp rofiles are inconsistent
in some importantr espectsw ith the measuredo nes. This exerts a major effect on the soot
predictioni n terms of the quantitatived etail in the computationw, hich is howeverc rucial for
the soot model development. The original flow field modelling needs to be improved for the
purpose of further soot model refinement

(1982). Computations of soot and NOx emissions from gas turbine combustor: final report&quot;

(1963). Convective mass transfer&quot;

(1993). Flow field prediction of NOx and smoke production in aircraft engines* Ph. D thesis,

(1962). Oxidation of carbon between 1000-2000 oC&quot;

(1979). Radiative heat transfer from gas turbine flames&quot;

(1988). readily obtained from Eqs. (C-2) and (C-3). Dilution zone tempgrature measurement An R-type thermocouple has been used which has a platinum shield of diameter d2&quot;2 I mm, the shield emissivity depending on platinum temperature (Ang et al.